1. Field of the Invention
The present invention relates to an apparatus for control of an internal combustion engine and to a device for predicting the amount of intake air. More particularly, the present invention is concerned with an apparatus for control of the fuel injection duration and the spark timing on the basis of the amount of throttle opening and the engine speed, and with an apparatus for predicting the intake air amount or a physical amount corresponding to the intake air amount around closing time of an intake valve used to control the fuel injection duration and the spark timing.
2. Description of the Related Art
In the field of internal combustion engines, there are known internal combustion engines where the fuel injection duration is controlled on the basis of the air amount passing the side upstream of the throttle valve or absolute pressure of the intake, or the absolute pressure of the intake pipe (hereinafter, referred to as the "intake pressure"), and the engine speed. The air amount and physical amount of the intake pipe both correspond to the amount of intake air taken into a combustion chamber of the engine. Thus, in an internal combustion engine, there are steps of calculating the intake air amount per rotation of the engine from these amounts and the engine speed, determining the basic fuel injection time from the intake air amount per engine rotation and on the basis of the air-fuel ratio, and determining the fuel injection duration by correcting the basic fuel injection duration in accordance with factors such as the intake air temperature, cooling water temperature, and so forth, and so controlling the amount of fuel injection by opening the fuel injection valve for a period of time equal to the thus determined fuel injection duration.
In this known system, when the fuel injection duration is controlled on the basis of the intake air pressure and the engine speed, the intake air pressure is, in principle, approximately proportional to the amount of intake air taken into the engine per cycle. A diaphragm type pressure sensor is attached to the intake pipe on the side downstream from the throttle, and the output from this pressure sensor is processed by a filter having a time constant of 3 to 5 msec for eliminating the pulsation component of the intake pressure caused by the operation of the engine. The basic fuel injection duration is computed from the thus detected intake pressure and the engine speed which is sensed by a suitable engine speed sensor. This known system has a drawback in that the detected change in the intake pressure has a certain time lag behind the actual change in the intake pressure during acceleration and other periods of transient operation of the engine. This delay is because of a delay in the response of the diaphragm of the pressure sensor, and due to a delay of response attributable to the time constant of the filter. Because of this, when the engine is being accelerated quickly by fast opening of the throttle valve accompanied by a drastic rise in the intake air pressure, the detected intake pressure rises rather slowly and so the basic fuel injection duration is computed on the basis of an intake pressure which is lower than the actual intake pressure. As a consequence, the air-fuel mixture supplied to the engine becomes too lean, resulting in the response of the engine to the acceleration demand being impaired, and in an increase in noxious exhaust emissions. Conversely, when the engine is being decelerated with fast closing of the throttle valve accompanied by a rapid drop in the intake air pressure, the basic fuel injection duration is computed on the basis of an intake pressure which is higher than the actual intake pressure. As a consequence, the air-fuel mixture supplied to the engine becomes too rich, resulting in the driveability of the engine being impaired, and in an increase in noxious exhaust emissions. In order to prevent these problems attributable to the generation of a mixture that is either too rich or too lean, various corrections have been performed, for example, using acceleration increments or deceleration decrements for the fuel supply. Nevertheless, because of the presence of the abovementioned time lag or delay in the detection of the intake pressure during transition operation of the engine, it has been impossible to control the air-fuel ratio of the mixture to the objective air-fuel ratio, over the entire range of engine operation.
Moreover, when the fuel injection duration is controlled on the basis of the air amount and the engine speed, the intake air amount is directly detected by a flow sensor such as a Kermin vortex type of air flow meter and an air flow meter mounted on the side upstream of the throttle valve. However, since the flow sensor is mounted on the side upstream of the throttle valve, a time lag occurs between the changes in the actual intake air amount and the corresponding changes in the flow sensor output. The result is the same problem as is described above.
Because of this, since the amount of opening of the throttle valve is a physical quantity having no time lag with respect to the actual intake air amount, fuel injection has been controlled on the basis of this amount of opening of the throttle valve, and the engine speed.
Japanese Patent Application Laid-Open Nos. 28031/1984, 96949/1984 and 122237/1985 propose that the basic fuel injection duration be determined using the amount of opening of the throttle valve of the engine, as a parameter that has no inherent time lag with respect to changes in the engine pressure. Japanese Patent Application Laid-Open No. 39948/1984 proposes that the basic fuel injection duration be determined by calculating the intake pipe pressure from the amount of opening of the throttle valve of the engine and the engine speed, and then using the intake pipe pressure so calculated, and the engine speed to calculate the basic fuel injection duration. The above described amount of opening of the throttle valve is detected by a voltage proportional to the amount of opening of the throttle valve and as output from a throttle valve opening amount sensor comprising a variable resistor comprising a contact fixed to the rotating shaft of the throttle, and in which one terminal is connected to a battery and the other to ground. However, throttle valves are normally located upstream from the engine combustion chamber(s) and as a consequence, a time lag is inevitably caused because a certain period of time is required for the air having passed the throttle valve, to reach the combustion chamber of the engine. Moreover, the phase of operation of the throttle valve is ahead of the phase of changes.in the actual suction of the mixture by the engine because of the volume of space in the intake pipe between the throttle valve and the intake valve of the engine. As a consequence, the phase of the intake pressure P(TA, NE) determined in accordance with the degree of throttle opening and the engine speed, is ahead of the phase of the actual intake pressure P, as shown in FIG. 24. Moreover, as shown in FIG. 25, the basic fuel injection duration TP (TA, NE) determined by the degree of throttle opening is greater than the fuel injection duration actually required because the phase of the change in the degree of throttle opening is ahead of the phase of the change in the actual intake air amcunt. Therefore, when the fuel injection duration is controlled on the basis of the degree of throttle opening and the engine speed, the actual fuel injection duration exceeds the that demanded during acceleration and the mixture is made excessively rich as a consequence. Conversely, during deceleration, the actual fuel injection duration becomes smaller than that demanded, and the mixture is made excessively lean as a consequence. When acceleration incrementation is performed for the fuel supply, the fuel supply rate is increased as shown by the hatched portion in FIG. 25, but the undesirable effects caused by the phase advance described above cannot be eliminated.
Moreover, the same problem as described above occurs because sparking is controlled on the basis of the degree of throttle opening and the engine speed.
Furthermore, the point at which the amount of air supplied to the engine combustion chamber is determined is the point at which intake is complete, or rather, the point at which the intake valve closes. Therefore, in order to control the values for the control quantities such as the fuel injection duration and spark timing, to those required by the engine, control of these control quantities can be performed using the values detected in the proximity of the intake valve opening valve at the point when the intake air amount taken in to the engine combustion chamber is determined, this is to say, when the intake valve closes. However, when fuel injection duration control is performed, because a certain amount of time is necessary to calculate the control quantities, a certain amount of time is necessary for the fuel injected from the fuel injection valve to travel to the combustion chamber after the intake air amount supplied to the combustion chamber is decided. Because of these delays, it no longer becomes possible to calculate and control the control quantities to the values required by the engine.
Therefore, in conventional apparatus such as Japanese Patent Application Laid-Open No. 157260/1987, the amount of change per unit time (Q.sub.n -Q.sub.n-1) .DELTA.T of the degree of throttle opening is determined and this amount of change is multiplied by the time difference .DELTA.T up till the point where the prediction is made, the degree of throttle opening is then calculated for this point and the results used as the basis for predicting the engine control quantities.
Nevertheless, as described above, the phase of operation of the throttle valve is ahead of the phase of changes in the actual suction of the mixture by the engine and, as a consequence, the phase of the control quantities determined by the degree of throttle opening and the engine speed is also ahead of the phase of changes in the actual suction of the mixture by the engine. Accordingly, even if the control quantities are predicted as in the conventional apparatus, by the amount of change in the degree of throttle opening, the fuel injection duration becomes greater than the rate demanded during acceleration and the air-fuel ratio becomes too rich, and the fuel injection duration becomes less than the rate demanded during deceleration and the air-fuel ratio becomes too lean.
Because of this problem, the applicant of the present invention has already proposed a method of controlling the fuel injection duration (Japanese Patent Application Laid-Open No. 51056/1987) using the engine speed and the degree of throttle opening with no response lag with respect to the actual intake pipe pressure, and using this as a basis to calculate the intake pipe pressure PMTA for the constant state and for performing time lag correction in transition states so that the current air intake pipe pressure PMCRT is calculated without phase lead or lag. This calculated air intake pressure is used as the basis for predicting the intake pipe pressure at the point where the air amount taken into the engine is determined, and then using this predicted value and the engine speed as the basis for controlling the fuel injection duration.
However, in the above stated proposed by the applicant of this invention, the intake air pressure is predicted by calculation only, for the point where the air amount taken into the engine is determined, and without taking into account the actual intake pipe pressure. Because of this, the accuracy of the predicted value is adversely influenced by discrepancies in the intake pipe pressure in the constant state, to produce the problem of irregular emission control.
Furthermore, if the atmospheric pressure changes, the air density changes so that the amount of air supplied to the combustion chamber changes even if the degree of throttle opening is maintained constant. This creates a discrepancy between the value required by the engine and the calculated value for the fuel injection duration and the resultant problem of irregular emission. This same problem also occurs in engines fitted with superchargers. In order to eliminate this problem, the intake air pressure can be measured and successive correction performed for the current intake pipe pressure PMCRT calculated on the basis of this measured value. However, the greater the discrepancy due to the atmospheric pressure, the higher the load and the accuracy of the values measured for transition stages deteriorates. This is illustrated by FIG. 26, for the situation where there is full acceleration at full throttle.
In the constant stage, intake pipe pressure PMTA discrepancy amount a, i.e., the true discrepancy amount of the atmospheric pressure becomes greater than the discrepancy amount b, i.e., the correction amount according to the above sequential corrections, so that the intake pipe pressure PMTA corrected using discrepancy amount b becomes smaller than the true value. As a consequence, the PMFWD value estimated using the intake pipe pressure PMTA after correction becomes smaller than the true estimated value, and the mixture is made lean.
Moreover, in engines fitted with superchargers, there is a blower provided to perform supercharging on the side upstream of the throttle. The pressure upstream of the blower therefore varies greatly in accordance with the conditions of operation and the intake pipe pressure PMTA and PMCRT vary as shown in FIG. 27. The same discrepancy shown in FIG. 26 is present even if the engine is fitted with a supercharger.
Furthermore, if the air amount flowing through the throttle is controlled by a bypass during idling to control the idling rotation speed, then when there are changes in the amount of air bypassing the throttle, the correspondence between the degree of throttle opening and the intake pipe pressure will deteriorate to cause a discrepancy between the estimated value and the actual value for the intake air pressure at the time of prediction, to result in the problem of not being able to control the control quantities to the values required by the engine.